Subnet Zero And All Ones Subnet

By | March 14, 2020

Subnet zero and all ones subnet are two different things buth both are co related. Subnet zero means we can use the network ID or network address ( ex. 192.168.1.0 ) as an IP address and we can configure it on our router interface.

All ones subnet means we can also use the broadcast ID or broadcast address ( ex. 192.168.1.255 ) as an IP address and we can configure it on our router interface as an normal IP address.

This feature is enabled on all the new IOS and it is default on all the IOS since 12.0 IOS. If you are still using an old IOS then you can also enable it using the ip subnet 0 global command.

In the past , network address and broadcast address were not allowed to be use as IP address. That time IEEE engineers was thinking that these address can only be used for network ID and broadcast ID. But later they allowed to use these addresses. But still it is recommended that these addresses should only be use on network devices like router or layer 3 switches as well as it should make sure that it will not cause any traffic.

In real environment , I have seen and configured these addresses on loopback interfaces as well as on physical interfaces

Below is the command to enable the Subnet Zero and All Ones Subnet on OLD devices:

Router(config)# ip subnet 0

Below is the example of both subnet zero and all ones subnet:

Subnet Zero And All Ones Subnet
Router# conf t
RouterA(config)# interface serial 0/0
RouterA(config-if)# ip address 192.168.1.0 255.255.255.0
RouterA(config-if)# no shut
RouterA(config-f)# exit
RouterA(config)# interface loopback 0
RouterA(config-if)# ip address 10.140.1.255 255.255.255.255
RouterA(config-if)# no shut
 
RouterC# conf t
RouterC(config)# interface serial 0/0
RouterC(config-if)# ip address 192.168.1.1 255.255.255.0
RouterC(config-if)# no shut
RouterC(config-f)# exit
RouterC(config)# interface loopback 0
RouterC(config-if)# ip address 10.170.1.255 255.255.255.255
RouterC(config-if)# no shut

In above example we are assuming that IOS is above 12.0. So we have not configured the ip subnet 0 command. But you can configure it to enable this feature.

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